Ultrasonic liquid treatment uses high frequency energy to cause vibration in liquids to produce physical or chemical effects. Ultrasound, part of the sonic spectrum that ranges from 20 kHz to 10 MHz, is generated by a transducer that converts mechanical or electrical energy into high frequency acoustical (sound) energy. The sound energy is then fed to a horn that transmits the energy as high frequency vibrations to the liquid being processed. A typical ultrasonic processing chamber is shown in Figure 3-1.

When liquids are exposed to these high frequency vibrations, both physical and chemical changes occur as a result of a physical phenomenon, known as cavitation. Cavitation is the formation, expansion, and implosion of microscopic gas bubbles in liquid as the molecules in the liquid absorb ultrasonic energy. Compression and rarefaction waves rapidly move through the liquid media. If the waves are sufficiently intense they will break the attractive forces in the existing molecules and create gas bubbles. As additional ultrasound energy enters the liquid, the gas bubbles grow until they reach a critical size. On reaching a critical size, the gas bubbles implode or collapse (Figure 3-2).

The energy that exists within the cavity and in the immediate vicinity of the gas bubbles just before collapse causes both physical and chemical effects in the liquid. Physical effects result when cavitation is intense enough to rupture cell membranes, free particulates from solid surfaces, and destroy particles and organisms through particulate collisions or by forcing them apart. Chemical effects result because the conditions immediately proceeding collapse of a cavitation bubble are similar in magnitude to ultra-high energy combustion conditions. Within the cavitation bubble and the immediate surrounding area, temperatures range from 2000 to 5000E C, and pressure reaches 1800 atmospheres. These extreme temperatures and pressures, which last only microseconds, do not exist long enough to heat the liquids being processed. However, the localized temperature and pressure increases are sufficient to increase chemical reactivity, polymer degradation, and chemical free-radical production (IES, 1998).

Because of the physical and chemical effects of the vibratory energy on liquid media, ultrasonic techniques are used in the processing of liquids for numerous applications. Current applications include emulsification, dispersion, disruption of biological cells, removal of trapped gases, cleaning of microscopic contamination, and acceleration of chemical reactions.

One of the parameters that influence the operational performance of ultrasound treatment is intake pressure. IES reports that pressures of 50 PSI are optimal for their 600-gpm model. However, they have not tested the effects of intake pressure at 70 PSI on optimal performance. It is their opinion it will not have much impact on operational and biological performance. Tests must be performed on the system to evaluate the effects of the increased pressure.

Ultrasound Treatment Performance

The primary means for biological eradication are the mechanical effects rather than chemical reactions that result from cavitation. Cell membranes and organisms are literally ruptured or blown apart from the intense energy created. The primary mechanisms of microorganism death are attributable to the physical effects associated with cavitation. These effects include:

• Completed destruction or death of microorganisms and larger biota, and

The vendor survey conducted under this study revealed that most current operating ultrasonic systems are used for small-scale flow applications (i.e., <100gpm). Only IES has developed and researched ultrasonic systems for applications and flow rates comparable to what is needed for commercial ship ballast-water treatment.

IES has researched ultrasonic effectiveness on microorganisms (both viral and bacterial) and larger organisms such as zebra mussels and Asiatic clams. Although IES has developed and installed 600-gpm systems for research purposes, and achieved good results, they have not installed a system for commercial use. The data presented in Table 3-2 represents information received by the IES for a 100 gpm-flow unit processing unfiltered water.

IES has no HPUP mortality data on potential nuisance species, such as Gymnodinium dinoflagellate cysts. However, HPUP and other ultrasonic treatments have demonstrated that sufficient energy and conditions are present to break zebra mussel shells. It is therefore speculated that ultrasonic treatment technology is capable of killing dinoflagellate cysts, one of the hardiest forms of biota in ballast tanks. Relatively simple laboratory tests would be necessary to test this hypothesis.

The biological effectiveness of ultrasonic technology will be enhanced to a certain degree by high level filtration to screen larger organisms. However, one of the primary mechanisms of organism mortality using ultrasonic technology is destruction or damage due to collisions within the liquid. Therefore, biological effectiveness is expected to increase with the amount of particulates in the water (O’Dette, 1998). Thus, a balance must be determined between the exclusion (by filtration) of the hardier larger organisms and the retention of some particulate matter to enhance the mortality of microorganisms in the ultrasonic treatment chamber.

Operational performance is not affected by prefiltration. Since the energy level in the processing chamber is high enough to keep particulate matter from settling, any particles introduced into the chamber during ballasting will be flushed out with the continual flow of ballast water being treated.

In summary, no determination can made be about the four pretreatment scenarios of 250-m m, 100-m m, 25-m m, and no filtration, nor to the expected biological effectiveness of current ultrasonic technology applied to ballast water.

In general, ultrasonic treatment systems (including the IES HPUP system) will be extremely resistant to typical shipboard conditions. There are no parts with friction surfaces to wear out. Since all parts of the system are fabricated or covered with stainless steel, equipment corrosion from saltwater is not a concern even if the systems are to be submerged in the ballast tanks. The generator tower, which would be more susceptible to adverse conditions, can be located in a different area than the treatment chambers.

Other conditions that the system maybe exposed to are high air temperatures, vibrations, rolling and pitching, and fumes. None of these will affect the efficiency or long term operation of the equipment.

The IES HPUP system discussed previously is relatively simple to operate. The system, requires operator input only at the generator tower. This input involves setting the controls at an initial setting and rechecking the controls when the unit is powered on. Some training is necessary to understand the control settings. Additional operational checks need to be performed on the system to ensure that the cooling water is circulating around the transducer.

Very little maintenance is required on the system and IES reports that their system can run for up to 12,000 hours before any maintenance is required. When maintenance is required, it involves approximately 2 hours of labor time to clean each unit of sediment residuals.

There are several ultrasonic safety concerns that should be addressed relative to shipboard application of this technology. The first issue is the use of high voltage (220 V) electricity to power the system. Considering that other shipboard machinery runs on high voltage (220 and 440V) this should not be a major factor assuming that shipboard safety requirements for high voltage electricity will to be adhered to during the installation process.

Heat build up in the transducer is the other safety concern. In the IES HPUP system, the transducer requires a circulating water-cooling unit whenever it is in operation. If the cooling system is operating properly, there should be no reason for safety concerns. All heated metal parts are out of contact and housed within the cooling unit. If, however, the transducer’s cooling system does not work properly, the transducer could overheat and fail. Risk from burning or explosions of the transducer are considered extremely remote.

There are no additives introduced into the ultrasonic system and no by-products generated by ultrasonic technology. Therefore, there are no anticipated environmental concerns associated with this technology.

The O&M costs associated with the IES HPUP system are minimal, with the exception of energy costs. Energy consumption for operation of the HPUP system is shown in Table 3-3. As stated previously, about two hours of cleaning maintenance is required every 12,000 hours of operation (for each 100-gpm-treatment chamber)